The invention relates to a communication system comprising a main station, a transmission medium, and terminals, said terminals comprising:
means for transmitting to the main station requests indicating a number of reservations to be granted to the terminal on the transmission medium,
a counter of requests for counting the reservations which have been requested by the terminal and which have not yet been granted,
means for calculating the number of reservations to be requested, taking into account the value of the counter of requests,
while said main station comprises a set of allocation counters, each associated with a terminal, for counting the reservations to be granted to the relevant terminal. The invention also relates to a terminal designed for use in such a communication system, to a requesting procedure designed for use in such a terminal, and to an updating method for the counter which is to be used in a main station according to the invention.
The invention finds its application, for example, in interactive communication systems by satellite or by radio waves.
Such a system is described in particular in the article xe2x80x9cCombined Free/Demand Assignment Multiple Access (CFDAMA) protocols for packet satellite communicationsxe2x80x9d by Tho Le-Ngoc and Jahangir Mohammed, published in the Proceedings of the 2nd IEEE International Conference on Universal Personal Communications, Ottawa, Canada, 12-15 Oct. 1993, vol. 2, pp. 825-828.
The requesting method described in this article consists in that the number of reservations which have been requested by the terminal and have not yet been granted to this terminal is monitored so as to calculate the number of reservations to be requested. This method is advantageous because it renders it possible to avoid a repeated reformulation of the same request when the initial request has not yet been granted. It is thus avoided that reservations are unnecessarily requested which are subsequently not used by the terminal. This renders it possible to optimize the shared utilization of the resources by the various terminals. This is particularly important in the case in which the transmission medium is a satellite channel because this type of transmission medium is particularly costly.
This method involves the assumption that the requests which have already been sent and for which no allocation has yet been received are in fact under treatment and will finally be honored. It is accordingly highly vulnerable as regards the loss of data which may arise from bad transmission conditions. In this method, in fact, the lost requests remain counted by the request counter, and the corresponding number of reservations is accordingly taken into account for all subsequent requests. The number of reservations requested by the terminal after such an incident is accordingly continuously lower than the actual need of the terminal. This has the result that the average waiting time for data before transmission is irreversibly increased.
The invention has for its object to remedy the above disadvantage. This object is achieved by means of a communication system as described in the introductory paragraph which is characterized in that said terminals comprise reset means for resetting their request counters and for transmitting to the main station an indicator for resetting the counter of allocations which is associated with the relevant terminal. Such a resetting serves to avoid that the terminal continuously counts reservations which have not been honored. This may relate, for example, to a periodic resetting.
In a more advantageous embodiment, said terminals comprise anomaly detection means and activation means for activating said reset means when an anomaly is detected. This embodiment is more advantageous because it serves to reduce the number of useless reset operations which each lead to an efficiency drop in the granting of reservations on the transmission medium.
The anomaly detection means comprise, for example, comparison means which compare the value of the counter of requests with a preset value. In another example, they comprise comparison means for comparing the value of the counter of requests at a moment t with the sum of the requests transmitted between said moment t and an earlier moment t-P, P being a period (called recovery period) which is sufficiently long for the reservations which were requested before the moment t-P to be granted at the moment t in the absence of an anomaly. If the value of the request counter at a moment t is higher than the sum of requests transmitted between t-P and t, this means that something irregular has occurred, for example that requests have got lost.